A thermal printer, which includes a thermal printer head composed of a heater element array extending in a width direction, is already commercially available. In the thermal printer, a thermal paper as a print paper is fed through between the thermal printer head and a platen roller rotating in contact with the thermal printer head. The thermal printer head performs printing while applying a constant pressure on the print paper.
In recent years, a portable thermal printer that can be held by a user has been developed as a thermal printer of the type described above. The portable thermal printer is easy to carry. For example, it can be carried to, e.g., a place of sale where a discount sale often called a special sale or a time-limited sale is underway and can be operated there to print any necessary information.
The portable thermal printer includes a drive power source with a driving gear arranged within a housing and a platen roller with a driven gear arranged in a cover for being able to open or close an opening portion of the housing. When the cover is kept closed with respect to the housing, the platen roller makes contact with a thermal printer head and the driven gear meshes with the driving gear.
Under these circumstances, the driven gear is exposed to the outside if the cover is opened. Thus, dusts or other alien substances are likely to adhere to the driven gear and may possibly be brought into between the driven gear and the driving gear, thereby hindering a smooth rotation of the platen roller. Accordingly, there is a strong demand for a portable thermal printer capable of assuring a smooth rotation of the platen roller.